Systems and methods for communicating with an implantable stimulator

ABSTRACT

An exemplary system for communicating with an implantable stimulator includes a coil configured to transmit a signal modulated with either on-off keying (OOK) modulation or Frequency Shift Keying (FSK) modulation. The system further includes a first telemetry receiver in the implantable stimulator configured to receive the signal in accordance with the OOK modulation and a second telemetry receiver in the implantable stimulator configured to receive the signal in accordance with the FSK modulation.

RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 11/043,404, filed Jan. 25, 2005 now U.S. Pat No. 7,822,480,which was a continuation-in-part of U.S. patent application Ser. No.10/607,962, filed Jun. 27, 2003, (now U.S. Pat. No. 7,177,698) whichclaims the benefit of Provisional Application Ser. No. 60/392,475, filedJun. 28, 2002. Priority is claimed to all of these patent applicationsunder 35 U.S.C. §120 or 119(e), and all are incorporated herein byreference in their entireties.

BACKGROUND

Radio-frequency (RF) powered implantable stimulators and battery poweredimplantable stimulators are described in the art. See, for instance,U.S. Pat. Nos. 5,193,539 (“Implantable Microstimulator); 5,193,540(“Structure and Method of Manufacture of an ImplantableMicrostimulator”); 5,312,439 (“Implantable Device Having an ElectrolyticStorage Electrode”); 6,185,452 (“Battery-Powered Patient ImplantableDevice”); 6,164,284 and 6,208,894 (both titled “System of ImplantableDevice for Monitoring and/or Affecting Body Parameters”). Each of thesepatents is incorporated herein by reference in its respective entirety.

Implantable stimulators configured to prevent or treat various disordersassociated with prolonged inactivity, confinement or immobilization ofone or more muscles are taught, e.g., in U.S. Pat. Nos. 6,061,596(“Method for Conditioning Pelvis Musculature Using an ImplantedMicrostimulator”); 6,051,017 (“Implantable Microstimulator and SystemsEmploying the Same”); 6,175,764 (“Implantable Microstimulator System forProducing Repeatable Patterns of Electrical Stimulation”); 6,181,965(“Implantable Microstimulator System for Prevention of Disorders”);6,185,455 (“Methods of Reducing the Incidence of Medical ComplicationsUsing Implantable Microstimulators”); and 6,214,032 (“System forImplanting a Microstimulator”). Each of these patents is incorporatedherein by reference in its respective entirety.

A typical implantable stimulator is intended to permanently remain inthe body of a patient once it is implanted. Hence, transcutaneouscommunication between an implantable stimulator and an external deviceis important for the stimulator to function properly. For example,communication with the implantable stimulator may be effected to performa number of functions including, but not limited to, transferring powerto the stimulator, transferring data to and from the stimulator,programming the stimulator, and monitoring the stimulator's variousfunctions.

SUMMARY

An exemplary system for communicating with an implantable stimulatorincludes a coil configured to transmit a signal modulated with on-offkeying (OOK) modulation to transmit control data. The exemplary systemfurther includes a first telemetry receiver in the implantablestimulator configured to receive the control data in accordance with theOOK modulation.

An exemplary method of communicating with an implantable stimulatorincludes modulating a signal with control data using on-off keying (OOK)modulation and transmitting the signal to the implantable stimulator.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate various embodiments of the presentinvention and are a part of the specification. The illustratedembodiments are merely examples of the present invention and do notlimit the scope of the invention.

FIG. 1 shows an exemplary implantable stimulator and an exemplaryexternal device according to principles described herein.

FIG. 2 shows a functional block diagram of an exemplary implantablestimulator according to principles described herein.

FIG. 3 shows a first signal including control data that has beenmodulated using frequency shift keying (FSK) and a second signalincluding control data that has been modulated using on-off keying (OOK)according to principles described herein.

FIG. 4 illustrates an exemplary OOK receiver that may be used to receiveand demodulate a stream of control bits that have been modulated usingOOK modulation according to principles described herein.

FIG. 5 illustrates an exemplary application specific integrated circuit(ASIC) implementation of an OOK receiver according to principlesdescribed herein.

FIG. 6 is a timing diagram of various signals corresponding to theexemplary OOK receiver shown in FIG. 5 according to principles describedherein.

FIG. 7 is a flow chart illustrating an exemplary method of communicatingwith an implantable stimulator using OOK modulation according toprinciples described herein.

Throughout the drawings, identical reference numbers designate similar,but not necessarily identical, elements.

DETAILED DESCRIPTION

Several types of implantable stimulators and external devices utilize amagnetic field to achieve transcutaneous communication via abidirectional telemetry link. An implantable stimulator and an externaldevice typically both have an RF coil that is used as the transmitterand receiver of the magnetic field. Accurate communication between animplantable stimulator and an external device typically requires aprecise reference clock within the implantable stimulator. The referenceclock allows timing synchronization in the transmission of data to andfrom the implantable stimulator so that the implantable stimulator maydemodulate data that has been modulated by the external device.

Many implantable stimulators include a precision circuit configured toprovide the precise reference clock. The precision circuit may requirecalibration data for the reference clock to be synchronized with thefrequency of the transmitting coil of the external device. Thiscalibration data may be transmitted by the external device via thebidirectional telemetry link. However, in some instances, thebidirectional telemetry link may fail due to a number of factorsincluding, but not limited to, a loss of battery power in thestimulator, interference, and/or coil malfunction. Without a functioningtelemetry link between the external device and the implantablestimulator, important control data such as, but not limited to, thecalibration data may not be transmitted to the implantable stimulator.

Hence, systems and methods for communicating with an implantablestimulator are described herein. A coil may be configured to transmit asignal including control data in accordance with a first telemetryscheme or a second telemetry scheme to an implantable stimulator. Theimplantable stimulator may include a first telemetry receiver forreceiving the control data in accordance with the first telemetry schemeand a second telemetry receiver for receiving the control data inaccordance with the second telemetry scheme. In some embodiments, thefirst telemetry scheme includes frequency shift keying (FSK) modulationand the second telemetry scheme includes on-off keying (OOK) modulation.

In the following description, for purposes of explanation, numerousspecific details are set forth in order to provide a thoroughunderstanding of the present system and method. It will be apparent,however, to one skilled in the art that the present system and methodmay be practiced without these specific details. Reference in thespecification to “one embodiment” or “an embodiment” means that aparticular feature, structure, or characteristic described in connectionwith the embodiment is included in at least one embodiment. Theappearance of the phrase “in one embodiment” in various places in thespecification are not necessarily all referring to the same embodiment.

FIG. 1 shows an exemplary implantable stimulator (10) and an exemplaryexternal device (20). The implantable stimulator (10) may be any type ofimplantable medical device, for example, an implantable microstimulator.Microstimulators are smaller than conventionally sized stimulators andare more easily implanted in a patient. Microstimulators may be injectedthrough a large bore needle or placed via a small incision in the skin.An exemplary, but not exclusive, implantable microstimulator is theBION® microstimulator (Advanced Bionics® Corporation, Valencia, Calif.)which may be configured to stimulate tissue to alleviate urinaryincontinence, reduce pain, or otherwise provide therapy for variousdisorders. Other examples of implantable stimulators include, but arenot limited to, spinal cord stimulators (SCS), cochlear implants, anddeep brain stimulators. As used herein and in the appended claims,unless otherwise specifically denoted, the terms “stimulator” and“microstimulator” will be used interchangeably to refer to anyimplantable medical device that may be implanted within a patient fortherapeutic purposes. A typical stimulator or microstimulator isconfigured to transcutaneously communicate with an external device.

The implantable stimulator (10) may be implanted in the target tissuearea of a patient and the external device (20) may be used tocommunicate with and/or transfer power to the stimulator (10). Suchcommunication and/or power transfer may include, but is not limited to,transcutaneously transmitting data to the stimulator (10), receivingdata from the stimulator (10), transferring power to a battery (16) inthe stimulator (10), and/or providing recovery power to the battery (16)when the battery has been depleted to zero volts.

As illustrated in FIG. 1, the implantable stimulator (10) may include anumber of components. A battery (16), which may be rechargeable, isconfigured to supply the stimulator (10) with power. A coil (18) isconfigured to receive and/or emit a magnetic field that is used tocommunicate with the external device (20). The coil (18) may also beused to receive energy used to recharge the battery (16). A stimulatingcapacitor (15) and two or more electrodes (22, 24) are configured tostimulate tissue with electric current. One or more of these componentsmay be housed within a case (not shown). The stimulator (10) may includeadditional and/or different electronic sub-assembly (14) configured toperform a variety of functions as best serves a particular application.

The functions performed by the external device (20) will vary as bestserves the particular application of the stimulator (10). The shape anddesign of the external device (20) will likewise vary. The externaldevice (20) may be embodied by the external components (20) shown inFIG. 1 of the present application's parent application (U.S. patentapplication Ser. No. 10/607,962). For example, the external device (20)may include a chair pad and a base station. In use, the chair pad may beplaced on a chair, and a patient who has an implanted stimulator (10)may sit on the chair pad to recharge the battery (16) in the stimulator(10) and/or to transfer data between the base station and the stimulator(10). The external device (20) may alternatively be a remote control orany other external component. In general, the external device (20) maybe any device configured to communicate with and/or transfer power to animplantable stimulator (10).

The exemplary external device (20) of FIG. 1 may include controlcircuitry (39) and an antenna/charging coil (34) configured to emitand/or receive a magnetic field that is used to communicate with theimplantable stimulator (10). The control circuitry (39) may be anycircuitry configured to control the operation of the antenna/chargingcoil (34). In some examples, the antenna/charging coil (34) and the coil(18) of the stimulator (10) communicate by sending RF signals across abidirectional telemetry link (48). The RF signals sent across thebidirectional telemetry link (48) may be modulated using a frequencydependent telemetry scheme, such as frequency shift keying (FSK), or bysome other modulation scheme. The antenna/charging coil (34) and thecoil (18) of the stimulator (10) may also communicate via a forwardtelemetry link (38). The forward telemetry link (38) may use an on/offkeying (OOK) modulation scheme. The forward telemetry link (38) is alsoknown as an OOK telemetry link. On/off keying (OOK) modulation isfrequency independent and is also known as pulse width modulation (PWM).The forward telemetry link (38) will be described in more detail below.

The external device (20) may be configured to perform any number offunctions via the bidirectional telemetry link (48) and/or the forwardtelemetry link (38). For example, the external device (20) may beconfigured to transcutaneously charge the rechargeable battery (16) inthe implanted stimulator (10). The external device (20) may also beconfigured to transcutaneously transmit data to the stimulator (10),receive data from the stimulator (10), and/or provide recovery power tothe rechargeable battery (16) when the battery (16) has been depleted tozero volts. The transmitted data may include configuration bits,programming bits, calibration bits, and/or other types of data.

FIG. 2 shows a functional block diagram of an exemplary implantablestimulator (10). As shown in FIG. 2, the coil (18) may be coupled to areceiver (42) and configured to receive an RF signal via thebidirectional telemetry link (48). The receiver (42) may be any circuitconfigured to receive and process an RF signal. For example, thereceiver (42) may be a microprocessor, digital signal processor (DSP),application specific integrated circuit (ASIC), processor with firmware,field programmable gate array (FPGA), or any other combination ofhardware and/or software.

The RF signal may be sent by the external device (20), for example, andmay include a carrier signal having modulated control data. The receiver(42) may then rectify the carrier signal to provide charging power forthe rechargeable battery (16) and demodulate the carrier signal toextract the control data. As used herein and in the appended claims, theterms “control data” or “control bits” will be used to refer to any dataor bits that are transmitted from the external device (20) to theimplantable stimulator (10) or from the implantable stimulator (10) tothe external device (20). For example, the control data may include, butis not limited to, calibration data used by a reference clock generationcircuit (56) and programming data used by a control circuit (58).

As shown in FIG. 2, the control data may be input into the controlcircuit (58). The control circuit (58) is configured to control theoperation of the stimulator (10). For example, the control circuit (58)may cause a pulse generator circuit (52) to generate and deliverelectrical stimulation pulses to a patient through the electrodes (22,24). The control circuit (58) may be a microprocessor, DSP, ASIC,processor with firmware, FPGA, or any other combination of hardwareand/or software.

In some embodiments, the coil (18) may also be connected to a backtelemetry circuit (54) to allow telemetry data to be sent from thestimulator (10) to the external device (20). The back telemetry circuit(54) may be any circuit configured to transmit data. The coil (18) mayalso be connected to an OOK receiver (43) to receive OOK modulated data.The OOK receiver (43) may be any circuit configured to receive andprocess an RF signal that has been OOK modulated. For example, the OOKreceiver (43) may be a microprocessor, DSP, ASIC, processor withfirmware, FPGA, or any other combination of hardware and/or software.Furthermore, the OOK receiver (43) may be integrated into the receiver(42). The function of the OOK receiver (43) will be described in moredetail below.

Referring to FIG. 2, the receiver (42) may require a precise referenceclock (ref clk) to rectify and/or demodulate the RF signal. The precisereference clock, as known in the art, may provide a clock signal havinga frequency that is synchronized with the frequency used by the externaldevice (20) to generate the RF signal. In some embodiments, the precisereference clock signal is generated by a reference clock generationcircuit (56). As shown in FIG. 2, the reference clock generation circuit(56) generates the precise reference clock signal based on a clocksignal generated by a clock generation circuit (55). The clock signal isalso known as the system clock. The clock generation circuit (55) may beany clock generation circuit known in the art and may output a clocksignal having an arbitrary frequency that remains constant over time. Inthe illustrated example, the reference clock generation circuit (56)includes precise resistor-capacitor (RC) networks and/or IC networksconfigured to process the clock signal output by the clock generationcircuit (55) and generate the precise reference clock signal.

The reference clock generation circuit (56) and the clock generationcircuit (55) are illustrated as separate components within thestimulator (10). However, it will be understood that the reference clockgeneration circuit (56) and/or the clock generation circuit (55) may beintegrated into a single component, integrated into the control circuit(58), or integrated into any other circuitry included in the stimulator(10).

In some embodiments, the reference clock generation circuit (56)requires calibration data to synchronize the frequency of the referenceclock signal with the frequency used by the external device (20) togenerate the RF signal. This calibration data may be periodically sent,for example, from the external device (20) via the bidirectionaltelemetry link (48). However, if the bidirectional telemetry link (48)is disabled for any of a number of reasons, some of which will bedescribed in detail below, the calibration data may not be conveyed tothe reference clock generation circuit (56) resulting in a referenceclock (ref clk) that is not synchronized with the frequency used by theexternal device (20). As will be described in more detail below, thefrequency independent OOK telemetry link (38) may then be used totransmit the calibration data to the reference clock generation circuit(56). The calibration data may then be used to resynchronize thereference clock (ref clk) with the frequency used by the external device(20) such that the bidirectional telemetry link (48) may again be usedto transmit control data and/or transfer power from the external device(20) to the stimulator (10).

The implantable stimulator (10) of FIG. 2 may operate in a number ofdifferent states. For example, the stimulator (10) may operate in anormal operational state wherein the stimulator (10) receives RF signalsfrom the external device (20) via the bidirectional telemetry link (48)and generates stimulation pulses. The stimulator (10) may enter into ahibernation state when the voltage output by the battery (16) fallsbelow a voltage defined by a battery voltage hibernation level internalregister (VHIB) (not shown). The VHIB is a programmable voltage value aof hibernation threshold for the battery (16). In the hibernation state,stimulation and FSK telemetry via the bidirectional telemetry link (48)are discontinued. However, in some embodiments, the stimulator (10)continues listening for an incoming OOK telemetry signal via the forwardtelemetry link (38). The stimulator (10) may also be configured todetect an applied external charging field used to charge the battery(16) while in the hibernation state.

The stimulator (10) may also operate in storage mode to conserve power.In storage mode, the stimulator (10) disables all circuitry except forcircuitry that is used to recharge the battery (16) and circuitry usedto listen for and receive an OOK telemetry signal via the forwardtelemetry link (38).

Hence, the OOK telemetry link (38) allows the external device (20) tocommunicate with the stimulator (10) even when the stimulator (10) isnot actively listening for an RF signal to be transmitted via thebidirectional telemetry link (48), e.g., when the stimulator (10) isoperating in the hibernation state or in the storage state. The OOKtelemetry link (38) also provides a communication interface between theexternal device (20) and the stimulator (10) that may be used inemergency situations, e.g., when the bidirectional telemetry link (48)fails or when there is an emergency power shutdown.

FIG. 3 shows a first signal (130) including control data that has beenmodulated using FSK and a second signal (131) including control datathat has been modulated using OOK, or PWM. The first signal (130) may betransmitted via the bidirectional telemetry link (48; FIG. 1) and thesecond signal (131) may be transmitted via the OOK telemetry link (38;FIG. 1). As shown in FIG. 3, the frequency of the first signal (130)varies between two frequencies, F1 and F2. FIG. 3 shows that a binary“1” is represented by the first frequency F1, and a binary “0” isrepresented by the second frequency F2. Alternatively, the firstfrequency F1 may represent “0” and the second frequency F2 may represent“1”. The first signal (130), as shown in FIG. 3, transmits the bits“1010110100” for illustrative purposes.

The second signal (131) of FIG. 3 has been modulated using OOKmodulation. As shown in FIG. 3, the signal (131) includes either a firstfrequency F1′ or no transmitted signal (frequency equals zero) for oneof two pulse widths, PW1 or PW2. A transmitted signal having a firstpulse width, PW1, regardless of whether the frequency is F1′ or zero(off), is interpreted as, e.g., a binary “0”; whereas a transmittedsignal having a second pulse width, PW2, regardless of whether thefrequency is F1′ or zero (off), is interpreted as, e.g., a binary “1”.Alternatively, a “1” may correspond to PW1 and a “0” may correspond toPW2. A change from the F1′ frequency to the zero (off) frequency is usedto indicate a data transition from one bit to the next bit in the datastream.

Thus, as illustrated in FIG. 3, the signal (131) first has a frequencyF1′ for a pulse width of PW1, indicating a binary “0”. The signal (131)then transitions to being “off” (frequency equal zero) for a pulse widthof PW1, indicating another binary “0”. The signal then transitions tohaving a frequency equal to F1′ for a pulse width of PW2, indicating abinary “1”. The rest of the states of the signal (131) are shown in FIG.3. Thus, the second signal (131), as shown in FIG. 3, transmits the bits“001001.”

As mentioned, the OOK receiver (43) of FIG. 2 may be configured toreceive and demodulate an OOK signal sent via the OOK telemetry link(38). In order for the OOK receiver (43) to distinguish between thefirst and second pulse widths (PW1 and PW2), a start bit having a fixedpulse width may be sent to the implantable stimulator (10) before anyother control bits are sent. The start bit may be a “0” or a “1”;however, for illustrative purposes, the start bit is a “0” in theexamples described herein. The OOK receiver (43) may be configured touse the clock generated by the clock generation circuit (55; FIG. 2) toincrement a counter for the duration of the fixed pulse width todetermine a “bit width threshold.” As will be explained in more detailbelow, the OOK receiver (43) may then compare the pulse widths of eachof the subsequent control bits in the OOK signal to the bit widththreshold and determine the values of the control bits.

Because the OOK receiver (43) compares pulse widths, the frequency ofthe clock signal generated by the clock generation circuit (55) does nothave to be synchronized with the frequency of the external device (20)in order for the OOK receiver (43) to function. Hence, the OOK telemetrylink (38) is considered to be “frequency independent” and may be used tocommunicate with a number of implantable stimulators (10) each havingdifferent system clock frequencies.

FIG. 4 illustrates an exemplary OOK receiver (43) that may be used toreceive and demodulate a stream of control bits that have been modulatedusing OOK modulation. As shown in FIG. 4, the OOK receiver (43) mayinclude a bit threshold counter (140) configured to measure the pulsewidth corresponding to the start bit. The OOK receiver (43) may furtherinclude a pulse width counter (141) configured to measure the pulsewidths of each of the control bits that are included in the OOKmodulated control data subsequent to the start bit. The clock signal(clk) generated by the clock generation circuit (55; FIG. 2) is inputinto each of the counters (140, 141) to facilitate the counterfunctions. The counters (140, 141) may be any type of counter circuitryknown in the art. For example, the counters (140, 141) may includeflip-flop circuitry.

As shown in FIG. 4, the bit threshold counter (140) generates andoutputs a value (A) representing the bit width threshold. The bit widththreshold may be equal to the pulse width of the start bit or,alternatively, the bit width threshold may be a multiple of the actualnumber of samples taken by the bit threshold counter (140). For example,the bit threshold counter (140) may output a value that is two times thenumber of samples taken by the counter (140) such that the bit widththreshold is equal to a higher value than the actual pulse width of thestart bit.

The pulse width counter (141) also outputs a value (B) representing thepulse width for each control bit included in the modulated control data.A comparator (142) may then compare the pulse width of each of thecontrol bits with the pulse width of the start bit. For illustrativepurposes only, the start bit is a “0”. Thus, if the pulse width of aparticular control bit is greater than the pulse width of the start bit,the comparator (142) outputs a “1”. Likewise, if the pulse width of aparticular control bit is less than or equal to the pulse width of thecontrol bit, the comparator (142) outputs a “0”.

FIG. 5 illustrates one exemplary implementation of the OOK receiver (43)of FIG. 4. The exemplary OOK receiver (43-1) illustrated in FIG. 5 isimplemented using an ASIC. However, the OOK receiver (43), as describedabove, may be implemented using any of a variety of differentcombinations of hardware and/or software. FIG. 5 shows that the counters(140, 141) may be implemented using digital flip-flops. The flip-flopsmay be D flip-flops (140, 141), as shown in FIG. 5, or they may be anyother type of flip-flop.

As shown in FIG. 5, digital circuitry may be used in connection with thebit threshold counter (140), the pulse width counter (141), and thecomparator (142). For example, logic (144) and AND gate (145) are usedto cause the bit threshold counter (140) to only measure the pulse widthof the start bit. Likewise, logic (144), inverters (146, 150),flip-flops (147, 149), and AND gate (148) are used to cause the pulsewidth counter (142) to measure the pulse widths of each of thesubsequent control bits in the control data. The logic (144) may be anycombination of digital logic as is known in the art. FIG. 5 also showsthat the OOK receiver (43-1) may include an edge detector circuit (143)that may be used to generate a clock signal (clk_ook) for thedemodulated control data (DATA_OOK). Exemplary VHDL (Very High SpeedIntegrated Circuit Hardware Description Language) code that may be usedto program the OOK receiver (43-1) is shown in the Appendix thataccompanied parent application Ser. No. 11/043,404, filed Jan. 25, 2005,which is incorporated by reference above. FIG. 6 is a timing diagram ofvarious signals corresponding to the OOK receiver (43-1) shown in FIG. 5and the VHDL code listed in the Appendix.

FIG. 7 is a flow chart illustrating an exemplary method of communicatingwith an implantable stimulator (10; FIG. 1) using OOK modulation. Asshown in FIG. 7, a start bit is first sent to the OOK receiver (43; FIG.4) (step 180). The pulse width of the start bit is then measured (step181). A control bit may then be sent to the OOK receiver (43; FIG. 4)(step 182). The pulse width of the control bit is then measured (step183). The pulse width of the control bit and the pulse width of thestart bit are then compared (step 184). If the pulse width of thecontrol bit is greater than the pulse width of the start bit (Yes; step184), a control bit having a value of “1” may be output by thecomparator (step 185). However, if the pulse width of the control bit isnot greater than the pulse width of the start bit (No; step 184), acontrol bit having a value of “0” may be output by the comparator (step186).

The preceding description has been presented only to illustrate anddescribe embodiments of invention. It is not intended to be exhaustiveor to limit the invention to any precise form disclosed. Manymodifications and variations are possible in light of the aboveteaching. It is intended that the scope of the invention be defined bythe following claims.

What is claimed is:
 1. A system, comprising: an external device,comprising control circuitry for producing from first data a firstsignal modulated with pulse width modulation, and for producing fromsecond data a second signal modulated with frequency modulation; and afirst antenna configured to wirelessly transmit the first modulatedsignal and the second modulated signal to the implantable medicaldevice; and an implantable medical device, comprising: a first telemetryreceiver in the implantable medical device for demodulating the firstmodulated signal to recover the first data; and a second telemetryreceiver in the implantable medical device for demodulating the secondmodulated signal to recover the second data.
 2. The system of claim 1,wherein the pulse width modulation comprises on-off keying (OOK)modulation.
 3. The system of claim 1, wherein the frequency modulationcomprises frequency shift keying (FSK) modulation.
 4. The system ofclaim 1, wherein the implantable medical device further comprises areference clock generation circuit for generating a reference clocksignal used by the second telemetry receiver.
 5. The system of claim 4,wherein the first data comprises calibration data used to calibrate thereference clock generation circuit such that the reference clock signalis synchronized with a frequency used to transmit the second modulatedsignal.
 6. The system of claim 1, wherein the first data comprises astart bit and a number of control bits, the start bit being transmittedbefore the control bits.
 7. The system of claim 6, wherein the firsttelemetry receiver comprises: a bit threshold counter configured tomeasure a pulse width of the start bit to generate a bit widththreshold; a pulse width counter configured to measure a pulse width ofthe bits; and a comparator configured to compare the measured pulsewidths with the bit width threshold to determine whether a bit comprisesa logic ‘0’ or a logic ‘1’.
 8. The system of claim 1, wherein theimplantable medical device comprises at least one electrode forstimulating a patient's tissue.
 9. The system of claim 1, wherein thefirst data enables the second telemetry receiver to receive the secondmodulated signal.
 10. The system of claim 1, wherein the first andsecond telemetry receivers are coupled to a second antenna.
 11. Thesystem of claim 1, wherein the first signal is used to communicate withthe implantable medical device while in the low power state.
 12. Asystem, comprising: an external device comprising a first coil coupledto control circuitry configured to transmit data in accordance with afirst telemetry scheme and a second telemetry scheme; and an implantablemedical device, comprising: a second coil, and telemetry circuitrycoupled to the second coil for listening for and recovering thetransmitted data whether transmitted in accordance with the firsttelemetry scheme or the second telemetry scheme, wherein the telemetrycircuitry discontinues listening for the data transmitted in accordancewith the first telemetry scheme when the implantable medical devicewhile in a low power state, and wherein the telemetry circuitrycontinues listening for the data transmitted in accordance with thesecond telemetry scheme when the implantable medical device is in thepower state.
 13. The system of claim 12, wherein the first telemetryscheme comprises a frequency shift key telemetry scheme wherein a binary‘1’ is represented by a transmitted signal of a first frequency, andwherein a binary ‘0’ is represented by a transmitted signal of a secondfrequency.
 14. The system of claim 12, wherein the second telemetryscheme comprises an On-Off-Keying (OOK) Pulse Width Modulation (PWM)telemetry scheme wherein a binary ‘0’ is represented by a first pulsewidth and a binary ‘1’ is represented by a second pulse width.
 15. Thesystem of claim 14, wherein a transition between one data bit and anadjacent data bit is marked by a change in a transmitted data signalfrom an ON to an OFF state or from an OFF to an ON state, wherein the ONstate is characterized by the presence of a data signal having afrequency, and wherein the OFF state is characterized by the absence ofthe data signal.
 16. The system of claim 12, wherein the telemetrycircuitry is bidirectional for allowing data to be sent to and receivedfrom the external device.
 17. The system of claim 12, wherein the firsttelemetry scheme employs radio frequency (RF) telemetry, and wherein thesecond telemetry scheme employs inductive telemetry.
 18. The system ofclaim 17, wherein the first telemetry scheme comprises a frequency shiftkey (FSK) communication scheme, and the second telemetry schemecomprises an On-Off-Keying (OOK) Pulse Width Modulation (PWM)communication scheme.
 19. The system of claim 12, wherein theimplantable medical device further comprises at least one electrode forstimulating the tissue of a patient.
 20. The system of claim 12, whereinthe telemetry circuitry comprises first telemetry circuitry forrecovering the transmitted data when transmitted in accordance with thefirst telemetry scheme and second telemetry circuitry for recovering thetransmitted data when transmitted in accordance with the secondtelemetry scheme.
 21. The system of claim 12, wherein the data comprisesa start bit and a number of control bits.
 22. The system of claim 12,wherein the second telemetry scheme is frequency independent.
 23. Thesystem of claim 12, wherein the low power state comprises one or more of(i) a hibernation state, (ii) a storage state, and (iii) an emergencypower shutdown.
 24. The system of claim 23, wherein the implantablemedical device comprises a battery, and wherein the hibernation statecomprises a state when a voltage of the battery falls below a threshold.25. The system of claim 23, wherein the implantable medical devicecomprises a battery, and wherein the storage state comprises a state inwhich all circuitry in the implantable medical device is disabled exceptfor circuitry used to recharge the battery and the telemetry circuitrywhich continues listening for the data transmitted in accordance withthe second telemetry scheme.